The present invention relates to a field decision correction apparatus used in video signal processing.
In video signal transmission of standard television system, a video synchronizing pulse signal comprising a horizontal synchronizing pulse, a vertical synchronizing signal and an equalizing pulse signal is inserted during a blanking interval of this video signal, and the video synchronizing pulse signal is separated from the video signal on the receiving side and a field decision is made to perform video signal processing.
Timing of field decision made by a field decision apparatus of the prior art will now be described by referring to FIGS. 2A and 2B.
With reference to FIGS. 2A and 2B, numeral 1 denotes a typical video synchronizing pulse signal, 2 a horizontal pulse produced from the above described video synchronizing pulse signal 1 by using a monostable multivibrator or the like to attain horizontal synchronization, and 3 a vertical pulse produced from the above described video synchronizing pulse signal 1 by using a monostable multivibrator or the like to attain vertical synchronization. Numeral 4 denotes a field pulse produced by decision made in a field decision apparatus of the prior art on the basis of the above described horizontal pulse 2 and vertical pulse 3.
The field decision apparatus of the prior art decides whether a field is odd-numbered or even-numbered by deciding whether the horizontal pulse 2 at a falling edge of the above described vertical pulse 3 is high or low and outputs the field pulse 4 which becomes a low level for an even field and which becomes a high level for an odd level.
In the above described video signal such as a video signal of special reproduction as in a household VTR (video tape recorder), uninterlaced information lacking in field information is inputted in some cases.
In this case, the result of the field decision is fixed to either of odd field and even field in the field decision apparatus of the prior art because of the lack of field information.
As a result, video signal processing which should be conducted repetitively on time in the order of odd field, even field, odd field,--is conducted for only either of odd and even fields. In a frame synchronizer, for example, field information is written into the memory of one of odd and even fields whereas information is not written at all into the memory of the other of odd and even field.
In reading out information from respective memories, information is read out alternately from the two memories. Therefore, a resultant image comprises an image successively changing read out from the memory in which new field information is written and a still image read out from the memory in which field information is not written. Thus two kinds of images are displayed. Regular images are not displayed, resulting in a problem.